Mike Holliday

Appendix: Realism and the Physical World

I want to look at the relationship between physics (especially quantum mechanics and cosmology) and the themes of emergence, existence, and explanation that I discussed in the main text. My purpose here is to see how these themes might affect our understanding of what modern physics is telling us about the nature of the universe. This is still an area of considerable puzzlement and debate; after all, the physicist Richard Feynman (1967, p. 129) once famously wrote 'I think it is safe to say that no one understands quantum mechanics.' So I don't pretend that I can present a clear picture of these notoriously murky waters. Nevertheless, there are some interesting similarities between the philosophical issues that I have been considering and the answers that physics is giving us to the question 'what is the universe really like?', and these similarities should make us wary of being led astray, led away from a clearer understanding of what physics can tell us, by our underlying philosophical assumptions. I shall consider this topic in an Appendix so as not to break up the flow of the main text.

1 The Quantum World

Let's start by considering the two-

Despite the strangeness of these results, they are in fact exactly as predicted by quantum theory. This is possible because the theory's formalism is such that it relates to the whole experimental set-

But I have not been quite right here. Quantum theory is in fact a mathematical formalism that relates future observations to earlier observations,3 and it therefore says nothing about the position of the photon until it is observed. The theory is simply an algorithm for calculating the probability that we will get a particular result when we make a measurement, in this case the position of the photon when it arrives at the wall of photoelectric cells. Given this, and the fact that we never actually observe a superposition (each photon hits a specific spot on the wall behind the slits), one obvious interpretation of what quantum theory is telling us is that position in space is not an inherent property belonging to all entities at all times, but is perhaps a form of emergent property -

It is interesting to note, however, that not all properties of sub-

One such puzzle is encountered in what are described as quantum eraser experiments. Variations on the two-

Delayed choice experiments have some similarity to the quantum eraser. For example, imagine that we could insert or remove the 45° polarizing filter very quickly, such that a decision as to whether to include it in the experimental set-

What such results seem to count against is the idea that each photon starts off in a delocalized state, and that its position only becomes definite when it interacts with the experimental apparatus. This doesn't seem to be a coherent account of what happens in the quantum eraser experiment, because in that case we add further measuring apparatus (the 45° polarizing filter), yet the result returns to what it was before we put in place the first set of filters; that is, we once again see an interference pattern. In a sense, this is not entirely surprising; thus far, I have been treating a particle's location in space as something that emerges in time, but this would seem at odds with notions that have been commonplace in physics since Einstein introduced his Special Theory of Relativity in 1905, in particular that space and time need to be considered as two aspects of a single space-

2 Time and Quantum Theory

There are two major reasons for believing that the way we think about time, in the everyday sense of the word, is not consonant with the world of modern physics. Firstly, special relativity strongly suggests that simultaneity is not a concept that reflects an underlying physical reality (see, for example, Saunders 2002). The effect of Einstein's theory on our conception of time can be seen from a recent poll of physicists, who were asked if they thought time was a fundamental concept, or whether it was derivative of something else -

On this view there is no more an objective division of the world into the past, the present, and the future than there is an objective division of a region of space into here and there. Not surprisingly, then, supporters of this view deny that there is any ontological difference -

There is no 'present', 'past', or 'future' in such a universe except relative to the frame of reference of an observer in that universe. I shall return to relativity, and more specifically general relativity, in a later section, but for now I want to discuss (at somewhat greater length) the second consideration that leads us to question our common conception of time, a consideration which is related to our perception that time is asymmetric in nature, in other words that the future is somehow radically different from the past. Much of the discussion in this and the following section derives from Huw Price's book Time's Arrow and Archimedes' Point (1996a), which contains a thorough analysis of the issues that are involved in trying to understand the true nature of this asymmetry.

The difficulty with taking time to be a fundamental element of the physical fabric of the universe is that it seems to be the case that physical processes are, at a micro level, time symmetric; that is, we cannot tell from observing such physical processes which way they are oriented in time. Consider two elementary particles colliding, and then view what happens in reverse -

I want to link this discussion of the asymmetry of time to two strands of thought that we encountered earlier. Firstly, Price's analysis shows the importance of not hypostasizing time as a metaphysical entity. Once we hypostasize time, then we make the asymmetry of time a fundamental feature of the universe. By doing so, we are unable to appreciate how this asymmetry may arise from other factors, and hence we block some of the possibilities for making progress in our understanding of the physical universe. Secondly, our discussion reveals once again the importance of an active observer (or more correctly, an active participant) in describing what we consider to be external reality. Huw Price has pointed out that, despite the tendency of physicists to think in terms of a block universe, there still seems to be a presumption that there is a sense in which the past is always determined and the future always undetermined. But, suggests Price, this could well be a view that we arrive at because of our existence at the macro level and the effect that the one-

Perhaps causal asymmetry isn't really in the world at all, but the appearance that it is is a product of our own standpoint. ... The difference between the fixity of the inputs from the past and the openness of the outputs to the future is a feature of the experience from the inside -

Now we can return to some of the puzzles of quantum theory. The asymmetry of time does not seem to be built into physical processes at the micro level, but instead appears to be due to the boundary condition of the early state of the universe. Accordingly, we seem to have no prima facie reason to think that events involving particles at the micro level will be correlated with past interactions but will not be correlated with future interactions. Indeed, quantum theory, and more specifically the experiments undertaken in connection with Bell's Theorem,9 suggest strongly that in some experimental situations the best explanation of the results is that physical variables (such as momentum, or spin) are correlated with measurements that we will perform on them in the future (Price 1996a, chapter 9).10

Huw Price describes this possibility by means of the phrase 'advanced action', which can be easily misunderstood as somehow involving causation directed backwards in time. But in the context of Price's earlier comments about the block universe, it seems more correct to understand the phrase as simply referring to the dependence of the present on the future (Price 1996a, p. 180). In a later article (1996b), Price clarifies his position:

The work done by [the advanced action] hypothesis in [quantum mechanics] does not require that these correlations be objectively causal. It simply requires that the correlations display certain distinctive patterns, the possibility of which has been overlooked by other approaches to [quantum mechanics]. (In particular, it requires that there be significant correlations between measurement settings and earlier states of measured systems.)

I interpret Price in general terms as holding that (i) time-

3 Entropy and Gravity

A few paragraphs ago, I left hanging the question why is the universe such that entropy always increases? After all, it is this fact that appears to lead to the asymmetry that we perceive in the universe at the macro level. Cosmologists are agreed that the very early state of the universe was exceptionally evenly distributed. As Price (1996a, p. 79) points out, we might think that there is nothing unusual in this; for example, we expect a gas to spread itself out evenly throughout a container. However, this is because the most important force in this case is the pressure caused by the movement of the gas molecules. In the case of the universe as a whole, gravity has a much more significant effect, causing matter to gather together; hence, we should expect a universe such as ours to be 'clumpy'. Price concludes (1996a, p. 101) that a smooth early universe is therefore a rather improbable state: if gravity provides a reason for the end state of the universe to be clumpy, then we have to explain why it doesn't provide a reason for the early universe to be clumpy also. One suggested solution to the puzzle of the smooth Big Bang is the possibility that, even if the universe starts at maximum entropy (i.e. complete disorder), its expansion allows increasing room for disorder and hence for pockets of structured order (see, for example, Stenger 1995, pp. 226-

This type of suggestion has been criticized by both Price (1996a, pp. 81-

This argument would be negated by a theory which provided an explanation as to why a smooth boundary at one end is reasonable and which also implied that there is only one such temporal boundary, i.e. that the universe does not contract.11 Unfortunately at this point I can offer nothing except outright speculation. A smooth universe is one where all the distances between particles are (nearly) identical. So I suggest that one possible way to identify a reason for the low level of entropy associated with the Big Bang is to consider a relational account of physical reality. We would ask ourselves the question 'what does it actually mean in a relational universe to say that distances are "different" or "the same"?' One implication might be that the separation between particles is definable in terms of other properties. If this were so, then the smoothness of the universe could be determined by properties that we associate with its age. So whether a stage of the universe is considered 'small' or 'smooth' may be closely related to other factors that belong to a universe that is in an 'early state', for example high temperature, whereas a 'later state' would tie together other aspects such as 'large' / high entropy / low temperature; (of course, this is only consistent with a universe that doesn't reverse its expansion and end in a so-

4 The Emergence of Space and Time

One of the most intractable problems in modern-

If general relativity is our modern-

Neither space nor time has any existence outside the system of evolving relationships that comprises the universe. Physicists refer to this feature of general relativity as background independence. By this we mean that there is no fixed background, or stage, that remains fixed for all time. In contrast, a theory such as Newtonian mechanics or electromagnetism is background dependent because it assumes that there exists a fixed, unchanging background that provides the ultimate answer to all questions about where and when (Smolin 2000, pp. 24-

Since space-

To describe the motion of a dynamical object, Newton had to assume that acceleration is absolute, namely it is not relative to this or that other dynamical object. Rather, it is relative to a background space. Faraday, Maxwell and Einstein extended the notion of 'dynamical object': the stuff of the world is fields, not just bodies. Finally, [general relativity] tells us that the background space is itself one of these fields. Thus, the circle is closed, and we are back to relationalism: Newton's motion with respect to space is indeed motion with respect to a dynamical object: the gravitational field (Rovelli 2001).

We can now perhaps obtain a clearer idea of the way in which the dynamic properties of particles can be considered 'emergent'. On Smolin's and Rovelli's account, there is no inherent background to the universe, and therefore it is not just position in space, but space itself which emerges from the relationships that constitute the universe. Similarly, general relativity also implies a relational account of time; it tells us about the way in which a number of variables evolve with respect to one another, but no single one of these variables is preferred in the sense that it represents 'proper time' (Rovelli 2000). Rovelli suggests that we instead consider time as having a statistical and thermodynamical origin, much as described by Huw Price.

Smolin views causality as the key relationship between the events that form the universe, but he is referring to causality not as some sort of metaphysical relationship but as a connection determined by the light cones that specify which events are in the past of certain other events. So described, causality is a structural feature of the relationships that comprise the universe. General relativity tells us that our universe is indeed a 'causal universe' in this sense, because no information or effect can travel faster than the speed of light, and it is this fact which restricts the size of the light cones of an event and hence determines that certain other events are in its 'past' and can be causally related to it:

So in our universe, specifying the paths of all the light rays or, equivalently, drawing the light cones around every event, is a way to describe the structure of all possible causal relations. Together, these relations comprise what we call the causal structure of a universe (Smolin 2000, p. 59).

In general relativity, the causal structure is itself dynamical in the sense that it is influenced by the events that it describes, evolving in accordance with equations formulated by Einstein. It is these equations that describe, inter alia, the curvature of space-

5 A Relational Interpretation of Quantum Theory

Another of the major mysteries of modern physics is the search for an intelligible interpretation of quantum theory; in other words, an answer to the question what does the universe have to be like for quantum theory to be correct?

The principal difficulty with quantum theory is that it does not appear possible to interpret it in a 'realistic' manner, such that atomic particles always have determinate properties such as position and momentum. It seems that these properties do not become definite until we try to measure them. An important distinction here is between the formal theory of quantum mechanics and its interpretation. Quantum theory is a methodology, an algorithm if you like, for generating various possible measurement results from an initial measurement(s); but the theory still needs to be interpreted, if we are to get beyond a description of sets of measurements. For example, suppose I have a source that emits an electron every minute, and I have a number of detectors set up in line to make measurements of the spin of the electrons in various directions. Then quantum theory simply provides a way of generating the probabilities of various spin measurements from detectors further down the line, based on the readings that I obtain from detectors nearer the source. Strictly speaking, all I am dealing with is measurements. Of course, there is a sense in which I have to make a minimal degree of interpretation when I say that these measurements relate to something called 'spin of an electron'. But interpretation proper is whatever takes me beyond the language of measurements; when I claim, for example, that after I make a measurement a particle has a spin of '1' in direction x. And a complete interpretation of quantum theory attempts to provide a coherent understanding of how all these measurements might relate to entities in the world, for example to properties of particles. But such an interpretation does not appear to be achievable .

The argument between Einstein and Bohr in the 1930s centered on whether it was actually possible to provide a realistic interpretation to quantum theory. Einstein argued that it was not possible to give such an interpretation and that quantum theory must therefore be incomplete. It was generally thought that Bohr had the better of Einstein in this debate, and the 'Copenhagen Interpretation' advanced by Bohr and others became the consensus opinion of physicists. However, there has been continuing dissatisfaction with Copenhagen, which is derided by some as merely an invocation to 'shut up and calculate', and the last fifty years have seen an increasing number of alternative interpretations.15

The difficulties seem to occur when we suppose that there are definite entities, e.g. 'electrons,' that have certain properties, e.g. 'spin of 1 in direction x'. The language that is used here is that of the world of 'medium-

Rovelli aims to derive the formalism of quantum theory from a small number of postulates that are motivated primarily from experimental results. Quantum mechanics, says Rovelli,

will cease to look puzzling only when we will be able to derive the formalism of the theory from a set of simple physical assertions ... about the world. Therefore, we should not try to append a reasonable interpretation to the quantum mechanics formalism, but rather to derive the formalism from a set of experimentally motivated postulates.

This methodology leads Rovelli to suggest that we reject something that is normally taken for granted -

The following example is used by Rovelli to show how we can more easily understand quantum mechanics once we have rejected this assumption. Suppose we have an observer, O, who makes a measurement of a system, S, the result of which can be either 1 or 2; also suppose that, when O actually makes the measurement, the result obtained is 1. Now assume another observer, P, who does not make a measurement of S, but who knows the initial states of both O and S. Once the measurement is made, O's description of S is that it is in state |1>, whilst P can only say that the combined system O plus S (i.e. the original observer and the original system) is in a superposition of two states, namely (i) S is in state |1> and O measures 1, and (ii) S is in state |2> and O measures 2. Recall from our earlier discussion of superpositions that this is most definitely not the same as saying that the combined system is either in state (i) or in state (ii). As Rovelli points out, this means that P's description does not include a specific statement that S is in the state |1>, which certainly does form an element of O's description. Therefore O and S have different descriptions of the state of S, and Rovelli thereby arrives at his main observation: 'In quantum mechanics different observers may give different accounts of the same sequence of events.'

Rovelli then considers the view that quantum mechanics is incomplete and that there is a 'deeper reality' that accounts for the relative nature of the descriptions made by O and by P. In fact, we see no experimental evidence suggesting that quantum mechanics is incomplete, and Rovelli's methodological suggestion is that we therefore accept that it is complete. This would imply that there is nothing in a complete description of the world beyond the information that systems have about other systems, and hence that physics is entirely relational. But of course, we anticipate that the descriptions given by O and P will be related to each other in some way. And we do indeed find that there is a relationship, but that this relationship is itself governed by the formalism of quantum theory: 'It is possible to compare different views, but the process of comparison is always a physical interaction, and all physical interactions are quantum mechanical in nature' (Rovelli 1996).

As an example, Rovelli considers one of the more intractable aspects of the interpretation of quantum theory, namely how to make sense of what is referred to as 'the collapse of the wave function'. The function that describes the probabilities of future measurement results evolves in a deterministic manner, except when a measurement is actually made, at which point it instantly changes so as to take account of the fact that this measurement must have had a definite result; the function now has to show that this actual result has a probability of 100%. So if the spin of an electron in a particular direction can be 1 or 0, then the function initially shows the state of the electron evolving (as, for example, it interacts with other entities or moves around in space) as a superposition of the two states, 1 and 0. However, once we actually measure the spin in that particular direction, we get a definite result, either 1 or 0. Suppose the result is 1: now the function suddenly changes so as to show that the result '1' has a 100% probability and the result '0' a zero probability. But there seems to be no way to account for this radical change in the function if the whole world is quantum mechanical in nature. The relational interpretation resolves this puzzle quite straightforwardly because the evolution of the state of a system assumes that it is isolated, an assumption that no longer holds once a measurement is made:

The unitary evolution does not break down for mysterious physical quantum jumps, or due to unknown effects, but simply because O is not giving a full dynamical description of the interaction. O cannot have a full description of the interaction of S with himself (O), because his information is correlation, and there is no meaning in being correlated with oneself. ... There is no way a system P may get information about a system O without physically interacting with it, and therefore without breaking down (at the time of the interaction) the unitary evolution description of O (Rovelli 1996).

But how are we to understand the 'physical meaning' of something having a value of a property only relative to something else? Rovelli says that it is possible to understand what is involved here if we consider how the matter looks to P, the second observer. Considering her description in quantum mechanical terms, we say that there is a correlation between the measurement expressed by O and the variable q that belongs to the system S. Suppose, for example, that the original 'observer', O, is a dial with a pointer. If the second observer, P, were to look at O and then take her own measurement of S, then the two results would be correlated. If P finds that S is in state |1>, then she finds that O's reading is 1; and if she finds that S is in state |2>, then she finds that O's reading is 2. The combined state of O plus S therefore contains information -

The existence of Planck's constant is interpreted by Rovelli as implying that the amount of information that can be extracted from a system is limited. When we combine this with a second postulate, that it is always possible to acquire new information from a system, then we can (almost) derive the formalism of quantum mechanics, thereby achieving the aim that Rovelli stated at the outset. Both these postulates are experimentally derived; they formalize what we have learned about the micro-

Rovelli notes that his views are closely linked to arguments made by Mermin to the effect that 'correlations have physical reality; that which they correlate does not'. If we stay with the correlations themselves, says Mermin (1998), then quantum mechanics is unproblematical: in order to fully describe the quantum state of a complex system, we only require the correlations between its subsystems:

It is a remarkable ... feature of the quantum mechanical formalism that all the joint distributions associated with any of the possible resolutions of a system into subsystems and any of the possible choices of observable within each subsystem, are mutually compatible: they all assign identical probabilities within any sets of subsystems to which they can all be applied. The physical reality of subsystem correlations need therefore not be restricted to any particular resolution of a system into subsystems or to particular choices of observable within each subsystem, even though different observables for a given subsystem fail, in general to commute. It is only when one tries to go beyond their inter-

Although all the correlations can be correct, they cannot all reflect determinate values of underlying properties since this produces inconsistencies. 'The correlata cannot all have physical reality because ... it is impossible to construct, in the standard way, a full and mutually consistent set of conditional distributions from the joint and individual subsystem distributions' (Mermin 1998). The distributions of probabilities referred to here are conditional because they refer to mutually exclusive situations; for example, we carry out experiment 1 or we carry out experiment 2, and we never get to carry out both experiment 1 and experiment 2 (at the same time on the same system).16

The relational interpretation of quantum mechanics displays the benefits that can be obtained by not assuming an ontology derived from our everyday existence. Rovelli's key move is to drop the assumption that there are such things as absolute properties of microphysical entities:

The core idea is to read [quantum] theory as a theoretical account of the way distinct physical systems affect each other when they interact (and not of the way physical systems 'are'), and the idea that this account exhausts all that can be said about the physical world. The physical world is thus seen as a net of interacting components, where there is no meaning to the state of an isolated system. A physical system (or, more precisely, its contingent state) is reduced to the net of relations it entertains with the surrounding systems, and the physical structure of the world is identified as this net of relationships (Laudisa & Rovelli 2002).

It is interesting that Rovelli's relational account accords an essential theoretical position to observers (in the sense of the term noted above) that is similar to that of Maturana and Varela as described in An Alternative Metaphor:

... states [of a system] have no absolute meaning, and must be interpreted as the content of the information that [another] system has about [them]. ... Thus, the properties of the systems are to be described by an interrelated net of observations and information collected from observations. ... There is no way to 'exit' from the observer-

This version started: October 2004

Last updated: July 2005

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Notes:

[1] This is not simply a property of light; in the last twenty years similar experiments have been carried out involving electrons and even whole atoms (see Gribbin 1995, pp. 7-

[2] Quantum theory describes the the state of a system, where the term 'state' is to be understood in a technical sense and not simply as a list of the values of all the properties of the system and/or the system's individual elements.

[3] For a good, basic introduction to the formalism of quantum mechanics, see Ismael 2004.

[4] I have some reservations about my use of the word 'universal' here. My philosophic instincts are too anti-

One advantage of thinking about the physical world in this manner is that we might be able to finesse the issue as to whether causation contains an element of necessity, an element that appears to be intimately involved in our understanding of causation and yet difficult to identify in the world. There seems to be a problem in thinking of causation as involving both a regularity of several sets of similar events (whenever A, then B) and a set of singular events (a causes b). This may appear less troublesome if the underlying similarity comes down to multiple manifestations (if this is the right word to use here) of a single physical constant in a space-

[5] The way in which our everyday concepts such as identity, objects, and space do not really apply at the level of the micro-

[6] Because of practical issues, actual experiments often utilize a beam splitter that separates light into two beams, rather than a single beam that then passes through two slits. The two resulting beams are then brought back together to display interference effects. In a delayed choice experiment, detectors are positioned in each of the two possible routes, and these can be switched on or off very quickly, quicker than the time it takes the beam to travel from the splitter. The idea is that we can choose to obtain an interference pattern (or not) after each photon has emerged from the beam splitter (see Baggott 2004, pp. 182-

[7] For a review of the philosophical support for the block-

[8] With one minor exception relating to the behavior of particles known as neutral kaons.

[9] Bell's Theorem is normally taken as proving that, given a small number of assumptions, any theory that reproduces the statistical predictions of quantum theory must be non-

[10] For a similar attempt to explain some of the difficulties of quantum theory by reference to the time-

Changing a detector off in another corner changes the experiment. Contextuality can be shown to be a natural feature that follows directly from a fundamental fact about quantum phenomena -

[11] Stephen Hawking once believed that he had proved a conclusion to this effect. Discussing the implications of his 'no boundary condition' cosmological theory, he says that that the theory itself can generate a direction in time: 'The universe is nearly homogeneous and isotropic when it is small. But it is more irregular, when it is large. In other words, disorder increases, as the universe expands' (Hawking 1994; quoted in Price 1996, p. 90). However, he now believes that his conclusion was mistaken; see Price 1996a (pp. 86-

[12] It is worth noting that Price (1996a, pp. 95-

[13] For non-

[14] It is true that general relativity is not generally held to support a relational view of physics. In particular, the development of field theory has strongly mitigated against traditional relational accounts of physical reality, such as those of Leibniz and Mach. Matter is now usually conceived of in terms of fields that range over a space (or space-

... the physical content is wholly unaffected by applying any smooth, invertible transformation (called a 'diffeomorphism') between spacetime points, which thus function as mere 'pegs' on which to 'hang' the fields ... This feature suggests that ... spacetime points should not be taken as real objects, even in interpreting classical general relativity: rather they are an artefact of the way we have formulated the theory. ... [The lesson is that] we should be wary of taking as the basic objects of our ontology (according to some theory) those items that are postulated as the initial elements in a mathematical presentation of the theory.

[15] These alternative interpretations include hidden variables (Bohm), many worlds (initially suggested by Hugh Everett), many minds, Cramer's transactional interpretation, decoherence (Zurek, Omnes), and proposals for adding to quantum dynamics (Ghirardi-

[16] Mermin's discussion (1998) does, unfortunately in my opinion, confuse the issue by attributing some aspects of the problem of the interpretation of quantum theory to human consciousness. For example, he says that '... physics can only (correctly) assert that photomultiplier #n firing is perfectly correlated with my knowing that photomultiplier #n fired for either value of n. The question that physics does not answer is how it can be that I know that it is #1 and is not #2. This is indeed a problem. It is part of the problem of consciousness.' I don't feel that this emphasis on consciousness is a particularly promising direction to take, although there are others who would disagree, for example Lockwood (1989) and Stapp (2004).